Development of the Human Zygote
November 16, 1995
Hundreds of thousands of times a year a single-celled zygote, smaller than a grain of sand, transforms into an amazingly complex network of cells, a newborn infant. Through cellular differentiation and growth, this process is completed with precision time and time again, but very rarely a mistake in the "blueprint" of growth and development does occur. Following is a description of how the pathways of this intricate web are followed and the mistakes which happen when they are not.
The impressive process of differentiation changes a single-cell into a complicated system of cells as distinct as bold and bone. Although embryonic development takes approximately nine months, the greatest amount of cellular differentiation takes place during the first eight weeks of pregnancy. This period is called embryogenesis.
During the first week after fertilization, which takes place in the Fallopian tube, the embryo starts to cleave once every twenty-four hours (Fig. 1). Until the eight or sixteen cell stage, the individual cells, or blastomeres, are thought to have the potential to form any part of the fetus (Leese, Conaghan, Martin, and Hardy, April 1993). As the blastomeres continue to divide, a solid ball of cells develops to form the morula (Fig. 1). The accumulation of fluid inside the morula, transforms it into a hollow sphere called a blastula, which implants itself into the inner lining of the uterus, the endometrium (Fig. 1). The inner mass of the blastula will produce the embryo, while the outer layer of cells will form the trophoblast, which eventually will provide nourishment to the ovum (Pritchard, MacDonald, and Gant, 1985).
Figure 1:Implantation process and development during
embryogenesis (Pritchard, MacDonald and
During the second week of development, gastrulation, the process by which the germ layers are formed, begins to occur. The inner cell mass, now called the embryonic disc, differentiates into a thick plate of ectoderm and an underlying layer of endoderm. This cellular multiplication in the embryonic disc marks the beginning of a thickening in the midline that is called the primitive streak. Cells spread out laterally from the primitive streak between the ectoderm and the endoderm to form the mesoderm. These three germ layers, which are the origins of many structures as shown in Table 1, begin to develop.
Table 1: Normal Germ Layer Origin of Structures in Some or all Vertebrates (Harrison, 1969)
Normal Germ Layer Origin of Structures in Some or All
Endoderm Skin epidermis
Hair Feathers Scales Beaks Nails Claws Sebaceous, sweat, and mammary glands Oral and anal lining tooth enamel Nasal epithelium Lens of the eye Inner earBrainSpinal cordRetina and other eye partsNerve cells and gangliaPigment cellsCanal of external earmedulla of the adrenal glandPituitary gland Dermis of the skinConnective tissueMusclesSkeletal componentsOuter coverings of the eyeCardiovascular system Heart Blood cells Blood vesselsKidneys and excretory ductsGonads and reproductive ductsCortex of the adrenal glandSpleenLining of coelomic cavitiesMesenteries LiverGall bladderPancreasThyroid glandThymus glandParathyroid glandsPalatine tonsilsMiddle earEustachian tubeUrinary bladderPrimordial germ cellsLining of all organs of digestive tract and respiratory tract
During the third week of development, the cephalic (head) and caudal (tail) end of the embryo become distinguishable. Most of the substance of the early embryo will enter into the formation of the head. Blood vessels begin to develop in the mesoderm and a primitive heart may also be observed (Harrison, 1969). Cells rapidly spread away from the primitive streak to eventually form the neural groove, which will form a tube to the gut. When the neural folds develop on either side of the groove, the...
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